Electrical installations run at medium or high voltage levels, such as transformers and in particular switches, are frequently operated with sulfur hexafluoride (SF.sub.6) as a quenching or insulating gas for such installations, which are appropriately sealed against leakage of SF.sub.6 gas.
The insulating materials in such installations must cope with relatively higher field strengths than in open or unsealed installations which do not employ SF.sub.6, as the superior stability or breakdown strength of SF.sub.6 as compared to air cannot otherwise be exploited. However, due to arcing or other forms of discharge and via the hydrolysis of decomposition products, as a result of the virtual impossibility of attaining an absolute seal against moisture, the SF.sub.6 can form cleavage products in such installations. Of such cleavage products, hydrogen fluoride (HF) can cause particular problems for the insulators.
Swiss patent specification No. 466,391 discloses the use of insulation bodies made from a cured casting resin, such as an epoxy resin, which is free of components, and in particular silicon compounds, which can react with the cleavage products of the insulator gas in an attempt to eliminate the problems caused by such products. For example, Al.sub.2 O.sub.3 in the form of technical grade corundum powder or in the form of clay is used instead of SiO.sub.2, which, particularly when in the form of quartz powder, is itself a very advantageous filler for casting resins.
Moreover, U.S. Pat. No. 4,102,851 discloses insulation bodies for use in gaseous SF.sub.6 environments which utilize a thermosetting resin matrix of a cycloaliphatic epoxy resin and aluminum hydroxide, Al(OH).sub.3, and/or natural magnesite (MgCO.sub.3) as an additive to the mineral filler consisting predominantly of extremely finely divided Al.sub.2 O.sub.3. The use of SiO.sub.2 as a mineral filler component is mentioned only in the case of a comparative example.
Furthermore, German Offenlegungsschrift No. 2,810,035 proposes using dolomite powder (MgCO.sub.3.CaCO.sub.3 in various, for example stoichiometric, proportions) as a filler in materials molded from an epoxy resin and used in installations which employ SF.sub.6, with the relative loss of strength resulting from the use of dolomite powder (as compared to SiO.sub.2 as a filler) being compensated to some extent by certain organic processing auxiliaries. The content of SiO.sub.2 which may be present in the mineral dolomite, however, should be below 1% by weight.
Finally, U.S. Pat. No. 4,104,238 discloses the use of a second mineral powder, aluminum hydroxide, Al(OH).sub.3, as a means for compensating for the disadvantages of using pulverulent SiO.sub.2 (in the form of quartz material) as a main component of the mineral filler used in insulation materials employed in gaseous SF.sub.6 environments. A hydantoin epoxy resin serves as the thermosetting resin matrix.
There are thus two methods in accordance with the state of the art for increasing the resistance of insulation bodies to SF.sub.6 cleavage products. In one method, SiO.sub.2 is virtually eliminated as a filler component of the thermosetting resin matrix and replaced by other mineral fillers. However, such fillers are comparatively expensive and/or result in a reduction of strength as compared to using SiO.sub.2.
In the other method, SiO.sub.2 still serves as the predominant mineral filler, i.e., it comprises at least half (.gtoreq.50%) of the mineral filler, with a second mineral powder component being used to increase the resistance of the insulation body to SF.sub.6 cleavage products. However, the effectiveness of aluminum hydroxide, proposed as the second mineral component in addition to SiO.sub.2, in increasing the resistance of the insulation body to SF.sub.6 cleavage products is limited.
Accordingly, the object of the present invention is to provide an electrical insulation article or body having increased resistance to SF.sub.6 cleavage products wherein a second mineral component is employed which contributes more effectively to increase the resistance of the insulation body to SF.sub.6 cleavage products, and in particular hydrogen fluoride.
This object is achieved in accordance with the present invention by employing from 5 to 50 percent by weight of the mineral filler, and preferably from about 10 to about 30 percent by weight of the mineral filler, of at least one alkaline earth metal carbonate as a second mineral component of the filler.